[T]here's a mantra that criticism must be constructive - that is, don't criticize someone if you cannot also tell them what to do about the problem.
I think that is a harmful norm, for two reasons. First, once a feature of your product is identified as weak, people other than the identifying party may come up with a solution. ... [E]ven if no one can think of a solution, having a weakness pointed out leads to a more realistic appreciation of your product, which by itself can be very helpful. When several people in your movie production company report vague feelings that a specific screenplay would not make a good basis for a movie if the aim is to make money, you may not want to produce it. If tests show that the brakes on a new car are not working properly, you may not want to start selling it. These are cases where nobody points out how to solve the problem, yet knowing that the problem exists is very important.